This article deals with ‘Oceanography: Ocean Salinity’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you canclick here
Introduction
Salinity of Solution is defined as amount of salt in 1000 gram of water .
Salinity of ocean water is 35.5 ppt ( parts per thousand) & maximum amount is of common salt.
Sodium
chloride
78%
Magnesium
Chloride
12%
Magnesium
sulphate
3.5%
Calcium
Sulphate
2.5%
Why Sodium Chloride (NaCl) is present in highest
proportion ?
Every salt has cycle &
they remain in sea water for specific time( called residual time) &
then precipitated to bottom surface .
Sodium (Na) & Chlorine
(Cl) has highest residual time in ocean water leading to very gradual
removal => that is why they are present in highest proportion .
Salt Budget
Irrespective of absolute
salinity, proportion of above salts
remain same in all parts of the world
.
Amount of addition or
extraction of fresh water compared to salt content in ocean water decides
absolute salinity of oceans .
Salt Budget = Budget of addition of salt &
removal of salt .
Sources of salts on ocean water.
Sediments carried by rivers
(most important) .
Submarine volcanism at Mid
Oceanic Ridge .
Chemical reaction between
rocks of geothermal vent of volcano & cold water.
Erosion of oceanic rocks and
wave erosion of coastal rocks .
Removal of Salts in ocean water.
Physical Removal : waves break at beaches ie salt spray .
Biological removal : marine
life forms extract calcium from sea water for their bones & shells .
Factors effecting salinity
Evaporation
Higher
the rate of evaporation ,higher is salinity.
Temperature
Warmer
parts are more saline than frigid ones.
Precipitation
Higher
the precipitation, lower is the salinity.
Influx of Freshwater
Influx
of freshwater leads to lower salinity .
Atmospheric pressure
Circulation of Ocean water
Stagnant
water has more salinity (Eg: Sargasso sea).
Windy situation
Wind
accelerate evaporation => windy situation = more saline
Salinity of Oceans
Standard salinity of ocean water is 35.5 ppt ie
salinity of Atlantic ocean .
Greater
than 35.5
High
saline.
Lower
than 35.5
Less saline.
Some highly saline lakes .
Man seldom drown in sea with high
salinity because water is highly
dense .
Dead Sea
238 ppt
West
Asia
Lake Van
330 ppt
Turkey
Great
Salt Lake
220 ppt
USA
Lake
Urmia
Iran
Overall pattern of Salinity across world
a. Latitudinal variation
Salinity is highest at tropics(not Equator) & decreases on both sides.
This is due to Interplay of
evaporation & precipitation & other complex interactions.
b. Hemispheric variation
Northern Hemisphere is warmer => high evaporation =>
more saline .
But southern Pacific => Roaring 40, furious 50 , Shreaking 60 , Screaming
70 => very fast winds . Hence in Pacific ocean , southern hemisphere
has more salinity
c. Local Variation in Salinity
Warm
ocean currents
– Increases temperature of water => equivalent to High evaporation . – Leads to more salinity.
Upwelling
Cooler water from depth come to surface => low salinity.
Transport
by currents
The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift.
d. Enclosed Seas
Tropical
region
– Warmer than open sea . – Will lead to high salinity. – Eg Mediterranean Sea , Persian Gulf etc.
High
Latitude
– Cooler than open sea . – Lower salinity than open seas . – Eg Baltic Sea, Gulf of Bothnia etc .
e. Inflow of large rivers
Ganga – Brahmaputra => flow into Bay of Bengal => large freshwater .
Bay of Bengal is less saline than Arabian Sea.
f. Glaciers
Those oceans /seas which receive greater glacier water are less saline
Baltic Sea is very less saline because of this reason .
This article deals with ‘Oceanography: Ocean Temperature’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you canclick here
Introduction
Temperature of ocean water is important because
Life of Phytoplankton & Zooplankton depend on it .
Affects climate of coastal lands .
Points to note :-
Main source of Energy on Earth is sun ie insolation .
Oceans play important role in energy & temperature regulation of earth. Because of its specific heat, water doesn’t get heated & does-not get cold very fast . Hence, water can store energy for very long time .
Average temperature of ocean from surface to bottom is 3-5 Celsius.
But average surface temperature of ocean is 25 Celsius.
Range of temperature
Daily/ Diurnal range of temperature
Difference between maximum
& minimum temperature of a day.
Daily range of temperature is
almost insignificant in case of rivers & is less than 1 degree C at
max
Tropical water has higher
diurnal range than equatorial waters because cooling & heating of
water is rapid under clear sky .
Annual range of temperature
Average is
12 degree C but lot of regional variation.
Higher in case of enclosed
oceans than open seas
Bigger the size lower annual
range of temp because of more mixing .
Distribution pattern of Temperature
Studied in two ways
1 . Horizontal distribution of temperature in oceans
Factors impacting horizontal distribution of temperature in oceans
a. Latitudinal Variation
There is decrease in temperature while going from equator toward poles because of decrease in insolation
But highest temperature is found not at equator but at tropics
Reason : High rainfall & cloud cover at the Equator resulting in High Albedo / reflection of sun-rays by clouds
b. Prevailing Winds
Direction
of the wind affects the distribution of temperature of ocean water.
Off shore winds blowing from
the land towards ocean or sea raise the temperature of ocean water.
Winds blowing from snow
covered regions in winter lower the surface temperature
c. Hemispheric Variation
Waters in Northern Hemisphere warmer than
Southern Hemisphere
. Reason is the oceans in the northern hemisphere receive more heat due to
their contact with larger extent
of land than the
oceans in the southern hemisphere.
Isotherms in north
aren’t regular while in Southern Hemisphere regular.
d. Enclosed Sea
Marginal seas of tropics ( like Mediterranean Sea) are warmer than open Ocean and marginal seas of temperate region (eg Gulf of Bothnia) are cooler than open sea.
Reasons
less mixing of water.
Land heats up surrounding waters too.
e. Ocean Currents
Warm currents
have warming effect on ocean water
Cold currents
have cooling effect on ocean water
f. Upwelling & Downwelling
Upwelling – Brings cool water from depth.
Downwelling – opposite ie warmer .
g. Salinity
If the sea water is more saline, its temperature will be higher because highly saline water contains more energy on the other hand the temperature of less saline water is low.
Horizontal distribution of Ocean Temperature
The average temperature of surface water of the oceans at equator is about 27°C and it gradually decreases
from the equator towards the poles.
The rate of decrease of temperature with
increasing latitude is generally
0.5°C per latitude
The average temperature is
around 22°C at 20° latitudes, 14° C at 40° latitudes and 0° C near poles.
The oceans in the northern hemisphere record relatively higher
temperature than in the southern hemisphere. The average annual temperatures for the
northern and southern hemisphere are around 19° C and 16° C respectively.
This variation is due to the unequal
distribution of land and water in the northern and southern hemispheres.
The highest temperature is not recorded at the
equator but slightly towards north of it.
2. Vertical Distribution of Temperature of Ocean Waters
Maximum temperature is on the
surface because it receives
insolation.
Transmitted to lower sections of ocean by
convection.
Solar rays can
penetrate very effectively till 200m
but seldom go below 1000 m . As a
result, temperature falls very rapidly from 200m till 1000m & after that , rate of decrease of
temperature is very slow (rate & not absolute temperature).
Can be divided into three zones
First layer
Top layer of warm oceanic water & is 200 m thick.
Temperature ranges from 20 to 25degree & no large variation with depth ( almost constant )
Present in tropics throughout year , in mid latitudes in summer & in cold areas never present .
Second layer
Thermocline layer
Below first layer from 200m to
1000 m.
Rapid rate of decrease of temperature to 4 C with increase in depth from 200 m to
1000 m .
Rate of decrease is rapid at
equator & tropics than towards pole because temperature already is
very low in Polar Region
In polar , waters are already at around 4C . Hence, this
layer is not present in polars .
Third layer
Very cold & extend upto
deep ocean floor.
Decrease in temperature with
depth is almost nill.
Polar areas have only this
layer from above till ocean floor.
Ocean Currents, Waves, Tides and Thermo-Haline Circulations
This article deals with ‘Oceanography: Ocean Currents, Waves, Tides and Thermo-Haline Circulations.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you canclick here
Introduction
Major movements of ocean water can be categorised as follows:-
1 . Horizontal Movement : Waves
The waves
are oscillating movements in the ocean water which transfer energy from place
to place.
Creation of waves
Wind pushes (away) the water
body & creates crest.
Gravity pulls the crest
downward & falling water pushes the former troughs upward .
Actual motion of water beneath wave is circular (& not straight) & while we go downward movement of
water molecules keep on decreasing.
Hence, in waves very slow
horizontal movement of water takes
place .
Waves actually involves the transfer of energy and water particles don’t move across the sea/ocean as
such. Water particles only moves in circular pattern as wave passes. Wind
causes waves to travel in ocean and energy is released at the shore.
2 . Horizontal Movement : Currents
Large mass of moving water from one part of the ocean to another in a definite direction is called as ocean current.
The movement is produced due to earth’s rotation, temperature difference of ocean water, salinity, density , air pressure and winds.
They circumnavigate the earth .
What Currents do
Heat re-distribution – from Equator to Poles.
warm water from equator to poles .
cold water from poles to equator.
Factors responsible for ocean current
The primary forces that influence the currents are:–
a. Wind
Most dominated force.
In most cases, direction of
wind will be direction of underlying current .
b. Insolation
Heating by solar energy cause
the water to expand. That is why, near equator the ocean water is about 8 cm
higher in level
than in the middle latitudes.
This causes a very slight gradient and water tends to
flow down the slope.
c. Salinity
Water will move from lower
salinity to higher salinity on the surface .
Reason : Water with higher
salinity being denser sinks & as a result water with low salinity
comes to fill the space.
Eg : Water from
Atlantic flows to Mediterranean
d. Coriolis Force
Coriolis
Effect : Due to rotation of earth – West to East .
Object moving in Northern Hemisphere will deflect Clockwise
Object moving in Southern Hemisphere will deflect Anti Clockwise
Movement of currents should be straight from equator to pole but not straight because
Coastline modifies the path.
Coriolis force
Nomenclature : Warm Current & Cold Current
Warm current is one that carry water
warmer than water found at that latitude .
Water flowing from low
latitude to higher latitude is warm.
Currents effect climate of bordering landmass eg wind blowing to
land from warm current cause rainfall. This is the reason why
mostly east coasts of continents receive rainfall & west coasts are
deserts.
General Pattern of Currents in all Oceans
Due to rotation of earth from west to east , two currents one on each side of equator are formed called North & South Equatorial Current.
Water carried in form of Equatorial currents tend to accumulate in western part of ocean or eastern coast of continent.
To balance this Counter Equatorial Current is formed between two Equatorial Currents from West to East.
Water at eastern coast of continent start piling up & under influence of density gradient start moving towards middle latitudes
At same time high density water at poles start flowing towards equator in form of currents along eastern coast of continent & two meet in middle latitudes where cold water sinks & warm water spreads on it .
The water is pushed towards east by strong westerlies & water reaches eastern part of ocean or western coast of other continent.
Obstruction at western part of continent divides water & one moves toward pole & other toward equator.
The water sinked in middle latitude upwells along the western coast to make good removal of water at equator . As a result circulatory cells known as gyres are formed one in each major ocean basin & is situated between 20 to 30 degree in both hemisphere.
This system is found in all oceans except Indian ocean where major changes occur due to presence of Indian peninsula & seasonal reversal of winds producing double gyre moving in seasonally opposite directions.
2.1 Currents in Atlantic Ocean
Warm Currents in Atlantic Ocean
a. North Equatorial Current
North equatorial current flows
from east to west between 5– 20 N latitudes.
After leaving the west coast
of Africa, it attains its main characteristics.
When it reaches the east coast
of South America, it splits into two branches and one branch called
Antilles current moves along the coast of West Indies and other branch is
diverted into the Caribbean sea.
b. South Equatorial current
It flows south of equator within 0 – 12 S
latitude in between the coast of Africa and South America.
It is stronger than the North
equatorial ocean current
c. Gulf Stream
Gulf Stream starts from the
Gulf of Mexico and carries warm waters into the colder latitudes.
It bends with the coastline up
to 40th parallel after which the direction is almost to the east, due to
the force and the direction of the westerlies and the deflective force of
the earth.
It joins the Labrador cold
current near New Found land, Canada
making it a good fishing ground.
d. Brazil Current
Warm current moving south
along the coast of Brazil.
It joins the Falkland cold
current near Bahia Blanca making it
a good fishing ground.
e. Norwegian Current /North Atlantic Drift
Norwegian Current takes warm water towards the coast of Norway allowing it’s coast to be operational throughout the year.
Cold Currents of Atlantic Ocean
a. Canary Island
It flows south along the Western coast of North Africa
between Maderia and Cape verde
Gradually , it merges with the
North equatorial current.
b. Labrador Current
In the north Atlantic, it
flows from the Baffin Bay and Davis Strait towards south.
It brings cold waters from polar zone and
moves along the coast of green land.
It meets with warm Gulf Stream
near New Found Land making it a good fishing ground.
c. Benguela Current
It flows northward along the
western coast of Africa
It carries cold waters from
sub-Antarctica surface water and mixes with south equatorial current
Side note : Newfound Land Island
Meeting point of Gulf Stream( warm current) & Labrador current (cold Current)
Amount of planktons increases resulting in large number of fishes. Hence, it is Good fishing ground.
Side Note: North Atlantic Drift / Norwegian Current
It provides warming effect to
western Europe.
Due to this, ports of Norway
– Narvik, Hammerfest , Murmansk etc keep on operating in
winter but at same latitude ports in Russia freeze during winter .
Side Note : Canary Current and Sahara Desert
Canary current is cold
current(passes near Canary Islands).
They give desiccating effect
(drying effect) to Sahara Desert resulting in no rainfall .
Side Note : Sargasso Sea
It is the only sea in the world without a land boundary. Its boundaries are defined only by ocean
currents. It is encircled by the Gulf
Stream in the west, north Atlantic drift in the north, Canary current in
the east and North Equatorial and Antilles current in the south
Due to confinement , Sargassum ie a sea weed grow in this region which obstructs
navigation. Plants have become adapted to life in the open sea, need no
roots or holdfasts for attachment, and are able to propagate by breaking
off bits to grow into new plants through vegetative propagation.
It is very saline. Reasons for this are
Prevention of mixing of fresh
water from surrounding oceans due to virtual boundary created by the
ocean currents.
High Evaporation due to high
temperature owing to its subtropical location
Gulf Stream brings in high
salinity water to the sea.
It is devoid of nutrients
& hence, poor marine life .
Side Note : Bahia Blanca & Falkland Island
Meeting point of warm Brazilian & cold Falkland Current .
Important for fishing
activity.
Side Note : Benguela Current & Namib Desert
Benguela Current is a Cold Current.
It gives desiccating effect leading to low rainfall on the south-west coast of Africa and formation of Namib Desert.
2.2 Currents of Pacific Current
Cold Currents of Pacific Ocean
a. Oyashio current
It originates from the Bering
Strait and flows towards south carrying cold water.
It meets with Kuroshio warm
current
b. California Current
Californian current is flowing
towards south along the west coast of U.S.A between 48 N and 23 N
latitudes.
Has desiccating effect which
leads to formation of Mohave and Sonoran desert.
c. Peru Current
Peru Current is the best
studied ocean current of the Pacific Ocean.
Alexander Von Humboldt in 1802
noted the details of the Peru Current. Hence, it is also known as Humboldt
Current.
It flows north along the west
coast of South America carrying cold water
d. Alaska Current
Cold current originating near Alaska and flowing south to meet North Pacific Current
e. West Wind Drift
It is an easterly moving drift
in the Pacific Ocean extending from Tasmania to the South American coast.
It is a cold current.
The speed of the drift is
greater under the influence of Roaring Forties.
Warm Currents of Pacific Ocean
a. North Equatorial Current
North equatorial current
originates from Revilla Gigedo island west of Mexico due to trade winds
and flows towards the Philippines Island covering a distance of about
12,000 km
It gets divided into two and
the northern branch joins the Kuroshio Current and the southern branch
abruptly turns and forms the Pacific counter current.
b. South Equatorial Current
South equatorial current is
originated due the action of the trade winds from east to west
It is stronger than the North
equatorial current.
It is further divided into
many branches due to the presence of many islands and uneven surface
topography.
c. Kuroshio Current
Warm ocean current flowing in
north easterly direction up to 30 N latitude
It meets Oyashio cold current
off the Kuril Islands.
d. East Australian Current
Warm current flowing southward along the east coast of Australia
Side Note : Japan is good Fishing Ground
Warm Kuroshio Current & cold Oyashio Current meets near Southern Japan. Hence, it becomes rich fishing ground.
Along with that Japan has very shallow continental Shelf leading to more photic zone and more marine organisms.
Indented coastline of Japan leads to formation of natural harbors .
Furthermore, Japan is a hilly region with less developed agriculture . Hence, there is main role of fishes in Japanese Diet.
Side Note : Peru Current and Atacama Desert
Peru / Humboldt current is cold current. Hence, it provides desiccating effect to the South American coast leading to formation of Atacama Dessert . Atacama desert is one of the driest places on Earth.
Current Topic : Pacific Garbage Patch
Pacific Ocean Garbage Patch is an enormous collection of marine debris (mainly plastic) that is collected and deposited by ocean currents in the middle of the North Pacific Ocean.
Reasons of
formation
Geographical
: Circular motion of the ocean currents draws debris into the center which is calm &
stable, where it becomes trapped.
Almost 80% of the debris in
the Great Pacific Garbage Patch comes from land-based activities in North
America and Asia&
the remaining 20% from boaters, offshore oil rigs, and large cargo ships.
The amount of debris in the patch accumulates since most of it is
non-biodegradable, mainly microplastics.
Impact
Destruction of marine life due
to ingestion of toxic and harmful
substances .
These toxic substances also
enters the food chain resulting in biomagnification
Presence of the garbage blocks the sunlight from reaching the planktons
and algae, thereby reducing productivity of marine ecosystem.
It affects the free flow of traffic through oceans.
Impacts the
tourism and
economic livelihood of small Pacific Ocean island countries
2.3 Currents in Indian Ocean
Current
system in Indian Ocean is unique & different from other current systems .
Why it is different than other currents?
In northern hemisphere, it is confined to tropical zone only & that too is divided by
Indian peninsula to form Arabian sea & Bay of Bengal.
Complete reversal in direction of winds as monsoon winds also impacts
the movement of current.
(wind is important factor – direction in which winds flow determine
direction of currents ).
Side Note : Navigation in Indian Ocean.
In Summer , Wind & current flows toward India .
Hence, sea vessel can move from Arabia to India .
In Winter, Wind & current flows toward Arabia .
Hence, sea vessel can move from India to Arabia .
Important for sea trade & cultural interaction since Ancient times.
Significance of Ocean Currents
Ocean Currents play an
important role in earth’s climate by distributing the energy within the
ocean. Along with that, they impact the local climate as
Warm ocean current increases
the temperature of an area
Cold ocean current decreases
the temperature of an area.
Currents also effect the
amount of rainfall.
Warm water currents increase
the rainfall because they bring winds along with them, laden with water
vapours. North America, Ireland,
Britain and south India receive rainfall because of this process.
On the other hand Atacama
desert remains dry because cold water currents don’t play any role in
occurrence of rainfall. Due to
this various deserts are situated on the western coast of Australia,
South America and Africa.
Currents helps in free
navigation navigation as Norweigian Current keeps ports of Scandinavia
navigable throughout the year and Kuroshio Current keeps ports of Japan
navigable during winter.
Conjugation of warm and cold
water currents helps in increasing the development rate of ‘Plankton’,
which is a diet of fish. This
conjugation takes place of eastern coast near New Found land in North
America, Bahia Blanca in South America, Japan coast in Asia etc. But on
the other hand conjugation of
currents forms dense fog which creates problem for marine navigation.
Sometimes ships are not able to move because of this fog. This whole situation effects the
economic activities of respective areas.
3. Vertical Motion of Ocean Water : Upwelling & Downwelling
Vertical circulation of ocean water takes place due to difference in salinity and temperature between the surface and the water deep below.
Upwelling
Upwelling is an oceanographic phenomenon that involves movement of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water.
UPWELLING & FISHING
Cool & nutrient rich water
come at surface.
Phytoplankton reproduce
rapidly in this condition => Consequently, fishes increase as well.
All Upwelling
Regions in World are good fishing ground. Eg : Canaries,
Benguela and Peru Coast
Downwelling
Down welling is the
process of accumulation and sinking of cold high saline water beneath
warmer or fresher water.
Not good for fishing .
4. Vertical Movement : Tides
What are tides
Rhythmic rise & fall in
level of ocean water.
Causes of tide formations (Prelims-2015) .
Moon’s gravitational pull is the most important
factor.
Sun’s gravitational pull ( although not as
prominent factor as Moon’ pull but still a major factor)
Centrifugal force due to earth’s rotation ( mainly responsible for
causing bulges).
On the
side of Earth facing the moon , a tidal bulge occurs while on the opposite side
although the gravitational attraction of the moon is less as it is farther
away, the centrifugal force causes tidal bulge on the other side
Stages of Tides
Stage I
Sea
level rises over several hours & cover the intertidal zone – flood
tide.
Stage II
Water
rises to highest level – high tide.
Stage III
Sea
level falls over several hours revealing intertidal zone – ebb tide.
Stage IV
Water
level stops falling – low tide.
Syzygy
Alignment of three or more
celestial bodies of same gravitational system in a line.
Effects tides in form of
variation between high tides & low tides.
a. Spring tide
Sun , moon & earth are in
line.
Occur on full moon or new moon (no moon day)
When there is greatest
variation between high tide & low tide .
High tide is higher than
normal high tide & low tide is lower than normal low tide.
b. Neap tide
Sun , Earth & Moon are at
right angle.
Occur when moon is at quarter
phase.
High tide lower than normal
high tide & low tide higher than normal low tides.
Daily frequency of Tides
Earth rotates & moon revolve in same direction, hence in relative term wrt to moon earth take 24 hrs & 52 minutes to complete one rotation.
2 high tides & two low tides in a day (24.52 hours to be exact)=Semidiurnal
Most enclosed water bodies & bodies away from open ocean experience only one high & one low tide .
Tidal Current
Movement of ocean water as a
result of tidal action is known as a tidal current.
In places of narrow coastal
inlet these tidal currents flow rapidly through the mouth with greater
height and velocity. For example in the Bay of Fundy, between Nova Scotia
and New Brunswick of Canada, the difference between high and low tides is
as high as 14m. Ports which utilize the tidal current for entry and exit
of ships from the harbour are known as tidal ports.
Importance of Tides
Creates inter-tidal regions which have special ecology of
their own. Eg : Sunderbans
Since tides are caused by the
earth-moon-sun positions which are known accurately, the tides can be
predicted well in advance. This helps the navigators and fishermen plan
their activities. Tidal flows are of great importance in navigation.
Tidal heights are very important, especially near rivers and
within estuaries having shallow ‘bars’ at the entrance, which helps ships and boats in entering into the
harbour.
Tides are also helpful in desilting the sediments and in removing
polluted water
from river estuaries.
Tides are used to generate electrical power (in Canada, France, Russia,
and China). A 3 MW tidal power
project at Durgaduani in Sunderbans of West Bengal is under way.
5. Thermo-Haline Circulations
Circulation because of the
temperature & salinity of the ocean.
Main driving force is freezing
of water at Atlantic and salinity .
Sequence
First of all water goes to
Polar Region ( Atlantic) . It interacts with the land which is very cold
and freeze . When water freezes, it will increase the salinity of
sub-surface water (because salt separates from water before freezing) .
Subsurface water will become
dense and goes further down. Consequently, it starts to flow as
sub-surface cold current. It will come to South Atlantic ocean and from
there to Indian and Pacific Ocean till the Bering Strait where it upwells
.
Then surface water starts to
flow back .
Note : This process is very slow and time frame between downwelling
and upwelling can take upto thousand years .
Impact of Climate Warming on this
Due to climate change , more
fresh water is being added in Atlantic . This will reduce the salinity
which is the driving force behind whole process.
Hence, climate change and
global warming will negatively impact this circulation
This article deals with ‘Oceanography: Relief features of Ocean.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you canclick here
Introduction
Hydrosphere
covers 70% part of Earth surface and is one of most important Spheres for
various living organisms and vegetation. Of the hydrosphere, 97.2% of water on
Earth is present in Oceans.
The oceans, unlike the continents, merge so naturally into one another that it is hard to demarcate them. The geographers have divided the oceanic part of the earth into five oceans, namely the Pacific, the Atlantic, the Indian, Southern ocean and the Arctic. The various seas, bays, gulfs and other inlets are parts of these four large oceans.
Oceanic Crust
Oceanic Crust is the most
fundamental crust.
It is Basaltic in nature .
Oceanic crust is more denser & darker than Continental Crust.
Age of continental shelfs is 1 billion year, oceanic crust
is younger & nowhere aged more than
200 million years .
Deepest point in ocean is
Mariana Trench (in Pacific ocean near Northern Mariana Islands with depth
10,971 m )
Oceanic Crust
is created at Mid Oceanic Ridge
& destroyed at Trenches.
Ocean floor
is not flat & more diverse than continents & quite different from
continental ones.
Oceans
consist of
71% of earth’s crust.
Division of Ocean
To study ocean bottom properly it should be divided into two divisions
Ocean Continent Margin.
Deep Ocean Plains.
1 . Ocean Continent Margin
Meeting point of ocean & continental crust . It
can be divided into three parts.
Coast / Coastal Plain
Continental Shelf
Continental Slope
Continental Rise
1 . 1 Coast
Coast can
be of two types
Submergent Coast : Estuarine , Ria , Fiord and Dalmatian
Emergent Coast
1 . 1 . 1 Submergent Coast
a. Fiord
Initially mountains are perpendicular to the coast . Glaciers come down to the coast forming u-shaped valley submerging the area. Hence, coastline which was initially straight becomes indented
Eg : Norwegian Coast
b. Ria
If valleys were submerged by rivers (instead of Glaciers) , it is called Ria
Eg : South Hampton, Wales,
British Isles etc
c. Dalmatian
If mountains are parallel to the coast (instead of perpendicular)
If ships are entering this
coast, they face barriers
Eg : Adriatic Coast (Italy)
d. Estuary
If normal low land region (not mountainous) is
submerged at the
mouth of river .
Eg : Thames, Buenos Aires
1 . 1.2 Emergent Coast
a. Lowland Emergence
It usually results in lagoon,
salt marshes, long beaches
Eg : Eastern Coast of India
b. Emergent Upland Coast
No natural harbors are found here
Eg : Scotland
1 . 2 Continental Shelf
Continental shelf is the seaward extension of land that lies under the sea water. It occupies 7% of the sea floor.
It is quite shallow (approx 200m ).
Region is called PHOTIC REGION ie receives large amount of sunlight . Marine organisms are found here (all floating planktons , benthos(crabs) & nektons(fishes)).
It has gentle slope with an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break.
Width varies – The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc. On the contrary, the Siberian shelf in the Arctic Ocean, the largest in the world, stretches to 1,500 km in width . (average width is 80 km)
The depth of the shelves also varies. It may be as shallow as 30 m in some areas while in some areas it is as deep as 600 m (Less depth means it will have more photic zone).
Made up of Granite rock ( because it is part of continental crust).
The continental shelves are covered with variable thicknesses of terrigenous sediments brought down by rivers, glaciers, wind, from the land and distributed by waves and currents. Massive sedimentary deposits received over a long time by the continental shelves, become the source of fossil fuels.
Continental shelves are well known for oil, natural gas, mineral deposits, sulphur, marine fishery etc. Large number of resources are found in continental Shelf.
a. Petroleum
90% of petroleum reserves of
world are found on continental shelves.
Because petroleum is formed
from submergence of marine
living organisms which get converted to petroleum under high temperature
& pressure (marine organisms like fish etc are maximum on Continental
Shelf because of Photic Zone)
Examples : Bombay High, Gulf
of Cambay, Persian Gulf, North Sea, Barents Sea(Russia) , Gulf of Mexico,
Norwegian Sea .
b. Sulphur
Rarely found on land .
Available in sea because
of marine
volcanism .
Gulf of Mexico is rich source of Sulphur.
c. Placer Deposit
Wave action erode beach rocks but there are some materials in rock which don’t disintegrate easily . Disintegrated material of lower weight is moved very swiftly by waves but heavy non-disintegrated material keeps on concentrating on shelf . These are called Placer Deposit.
Example are as follows
Monazite Sand found on Kerala Coast which is source of Thorium . It is important for Indian Nuclear Program
Gold in Alaska .
Zircon on coast of Brazil & Australia.
Diamond on South African Coast .
d. Calcium
Least soluble in ocean water
& can be easily extracted .
Peruvian
coast is rich in Calcium &
Phosphate .
e. Fishes
Due to photic zone and
availability of planktons , fishes are also found in the wasters of
Continental Shelf.
World famous fishing grounds
like Grand Bank are situated here
f. Pearls
Pearls of ornamental value are also found here.
Side Topic : Sea Boundaries (under UNCLOS)
a. Internal waters
All the water that is inside the coastline is called Internal Waters
State has complete sovereignty
over this and it is equivalent to
territory of country
b. Territorial Sea
Upto 12 Nautical Miles (1 NM = 1.85 Km)
(provided there is no other country on the other side. If any other
country is present, it becomes midway)
Here too almost all of the
laws of the land apply. It is almost as good as territory
Vessel of any other nation
need permission to enter into territorial waters of other country &
submarine has to come on surface & declare its identity while
navigation .
c. Contiguous Zone
Upto 24 Nautical Miles .
Some dilution of political
laws
Navy & Coast Guard cant
guard this but if criminals like smugglers after doing some crime try to run then that
country can follow them till this point .
d. Exclusive Economic Zone (EEZ)
Upto 200 Nautical Miles.
All the economic
laws of country apply but political
laws don’t apply here
Country can explore both biotic and abiotic
resources of the water and ocean floor .
Other countries have right to
peaceful passage .
Exceptional Case
If Continental shelf is beyond 200 Nautical Miles , country can claim EEZ upto that point with maximum limit of 350 NM
Within this extended zone, state has exclusive right of exploiting and exploring non-living resources of the sea bed as well as sedentry species on the sea bed. Fishing rights arent inclusive in 200 to 350 Nautical Miles.
d. International waters / High Sea
Beyond EEZ
Here resources belong to
everyone. No nation has sovereign rights over this area.
Side Topic : Nautical Mile
It is the distance
per one
minute of latitude (1/60th of 1 degree)
1 Nautical Mile = 1.85 Km
(1.85*60=111 Km = Distance
covered per degree latitude . )
At equator, 1 degree of longitude is also 111 km . But this distance
keep on reducing while going North and South and is 0 at poles . But distance
between latitudes is always same (because they are parallel) and is 111 km
approx at every point
1 . 3 Continental Slope
The zone of steep slope extending from the continental shelf to the deep sea plain or abyssal plain is called continental slope.
It has very steep slope .The slope angle varies from 5° to 60°.
No deposition is possible because of steep slope . Hence, no resources are found in this zone.
But some unique landforms are created specially when there is mouth of very large river at continental shelf like Indus, Ganga, Mississippi, Yukon & Congo. Example of such landform are
SUBMARINE CANYONS in which flow of water would be so great that it will vertically erode both continental shelf & slope . In Indian Ocean, submarine canyons are found at mouths of river Ganga and Indus.
Underwater Waterfalls – Highest waterfall on earth is under the Denmark strait.
1 . 4 Continental Rise
Transition between Continental
Slope & Sea Floor.
There would
be no continental rise in case there is TRENCH after Continental
slope .
2 . Deep Ocean Plains
Major portion of Ocean Crust consist of Deep Ocean Plains which have various relief features like Mid Ocean Ridge, Islands, Guyots etc. These are as follows :-
2 . 1 Abyssal/ Deep Sea Plains
Occupies 40% of ocean floor.
Consists older parts of oceanic crust that are smoothened
due to sediment deposition.
Among flattest, smoothest
& least explored regions of earth.
Average depth varies from 3000
to 6000 metres .
It has deposits from
continents( terrigenous), marine life(biogenous) & salts and
minerals(inorganic).
Abyssal plains of Southern
Indian Ocean and Eastern Pacific Ocean are rich sources of Poly Metallic
Nodules.
Side Topic : Poly-Metallic Nodules (PMN)
Polymetallic Nodules (AKA Manganese Nodules) .
Multiple metals are concentrated around core .
Found at depth of around 4,000 to 6,000 m on the floor
Were first found around Kara Sea & Arctic sea but later nodules were found in all oceans .
But are economically viable to extract only in Central Indian Ocean & Eastern Pacific Ocean.
Composition : Manganese (Highest) and apart from this contain Iron , Silicon, Al, Nickel & Cobalt & some rare metals.
Important for India as India doesn’t have good source of Nickel & Cobalt .
Countries are in race to explore & extract PMN.
2. 2 Islands
An island is a landmass surrounded by water on all sides.
They are of four types
a. Continental Island
Once part of large continent.
Examples : Sri Lanka,
Madagascar, Vancouver, Baffin islands.
Note : Madagascar Island formed after separation from Indo-
Australian Plate
Generally bigger in size than
all other categories of Islands.
Rock composition and all of
continental island is same as plate from which it is separated .
b. Volcanic Island
Ocean – Ocean plate collision
or Hotspot activity.
Marine organisms, the coral
polyps colonize the tropical warm water and form islands known as coral
islands.
Examples include Lakshadweep,
Maldives, Mauritius, Bahama & Bermuda
d. Sandbar Island
Formed by accumulation of sand
near coast => Sand which is eroded from main land is again settled near
coast & if continental shelf is shallow nearby then sandbar island is
formed .
Example : Long Island (New
York)
Side Topic : Archipelago
Cluster of islands .
(GK) Largest archipelago.
By area
Indonesia
By number of islands
Finland
2.3 Coral Reefs
Corals are indicative of health of Marine Ecosystem ie if coral reef exists at any place in healthy way , entire ecosystem is healthy.
Wherever there is coral reef , you will find great biodiversity at that point.
Most diverse ecosystem of world => 10% area but 25% marine species . They can be said to be Rain Forests of the Ocean.
What are corals?
Corals are tiny fleshy Sea Anemones Polyps.
They extract Calcium from sea
water for their skeletons to protect their body.When one generation die
other generation comes on it.
Corals live in symbiotic relationship with microscopic algae- Zoo-xanthalae which have photosynthesis
capabilities.
Zoo-xanthalae provide food to
corals and corals provide protection to zooxanthalae.
Conditions conducive for corals
Water temperature in
the range of
22-29 degrees
Celsius. Thus,
areal distribution of
corals is limited to tropical
and sub-tropical zones.
They will
not flourish in
areas of cold
currents but thrive
in warm current
regions. Therefore, they
are generally absent
on western coasts
of continents.
Depth
of water should
not exceed 180 feet
as the sunlight
is too faint
for photosynthesis which
is essential for survival
of microscopic Zoo-xanthalae
on which coral
polyps depend. However,
there should be plenty of
water as polyps cannot survive for too long
out of water.
Water should be
salty and
free of sediments.
Corals are best
developed on seaward
side of reef,
where constantly moving waves, tides and currents maintain an abundant supply
of clear, oxygenated water
Spatial Distribution of corals in the world
Spatial distribution of Corals in India
In India
they are found in
Gulf of Mannar.
Andaman &Nicobar
Lakshadweep
Gulf of Kutch
Sindudurg coast ( near Malvan
Coast) in Maharashtra ( only here
in India, bleaching hasn’t happened yet)
While
the Lakshadweep reefs are atolls, the others are all fringing reefs.
Coral Bleaching
When corals are in stress they expel Zoo-xanthalae.
Zoo-xanthalae give the colourful appearance to corals . In their absence, corals seems white (their original colour) .
Ultimately without food corals will die out too.
Reasons of coral Bleaching
Global Warming : increase in sea surface temperature .
Ozone Depletion : increase in solar irradiation.
Ocean Acidification : because of increase in Carbon Dioxide in atmosphere . This Carbon Dioxide dissolves in ocean water & water becomes acidic . Corals can’t live in acidic water
Algal Bloom : excessive use of fertilisers go to ocean through rivers resulting in Algal Bloom . Sunlight cant go below & corals die.
Increase in sedimentation from rivers.
Marine pollution like oil spills . No sunlight & oxygen reach below.
Diseases.
Coastal construction and shoreline development: Results in heavy sedimentation
Coral mining: Live coral is removed from reefs for use as bricks etc.
Consequences of Coral Bleaching
It will impact marine
ecosystem, as coral reefs are some of the most bio diverse and productive ecosystems.
Reefs act as natural barriers to shorelines, protecting them from the
effects of moving water. As coral reefs die, coastlines become more susceptible to damage and
flooding from storms, hurricanes, and cyclones.
Ocean will not be able to absorb as much
Carbon Dioxide
because Zoo-xanthalae
absorb Carbon Dioxide for photosynthesis leading to increased Global Warming.
Measures need to be taken
Political measures: The global agreements combating climate change must
keep the needs of coral islands as top priority.
Switching to sustainable livelihood measures: It is high time to recognize
the carrying capacity of each coral island ecosystem and limit population
growth, tourist foot fall etc. at these islands.
Sustainable development : Avoid major projects in threatened areas. Eg :
Carmichael Coal Mine in Australia that threatens Great Barrier
Reef should be avoided.
Formation of Coral Reef
Coral Reef
can be developed both at margins of continents or around islands .
3 stages of coral formation are as follows
Stage 1 : Fringing
Coral reef starts to develop along margin
of an island
& subduction of island just starts
First stage of Fringing Reef
Stage 2 : Barrier Reef
Corals keep on accumulating
and keep on rising upwards. As a
result, large part of island is submerged in sea/ocean.
Lagoon formation occurs
Stage 3 : Atoll
Last Stage
Island is submerged & only ring remains visible .
Note : In India, while the Lakshadweep reefs are atolls, the others are all fringing reefs.
Side Topic : Great Barrier Reef , Australia
Adani’s are investing in Coal Fields in Australia (Carmichael Coal Fields) . Project will create jobs for
Australians and coal will be imported to India
But due to large
Carbon-dioxide emissions in the area
, Great Barrier Reef will be impacted heavily.
2.4 Oceanic Trench/ Oceanic Deep
The long, narrow, steep-sided
depressions formed along the
convergent boundary where one plate subducts below the other is called Ocean trenches.
The Challenger
Deep in the Mariana Trench, (10,994 m) in the Pacific Ocean
is the deepest part of the earth.
Most trenches are in Pacific ocean & they are most active geological feature. Great earthquakes
leading to Tsunamis occur here.
As many as 57 deeps have been
explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic
Ocean and 6 in the Indian Ocean.
Main Trenches of the world
2.5 Seamounts / Guyots
Flat topped volcanic hills submerged under the sea water are called guyots.
Seamounts are conical, volcanic hills submerged
under ocean water
Seamounts , guyots and the water column above them serve as
important habitats, feeding grounds and sites of reproduction for many
open-ocean and deep-sea species.
2.6 Abyssal Fans
Also known as Deep Sea Fans or Underwater Deltas or Submarine Fans .
Delta like structure formed at deep sea surfaces . Major reservoirs of biodiversity.
2.7 Mid Oceanic Ridge
Mid-ocean ridges are submarine mountains.
Mid oceanic ridges are located on the divergent plate boundaries where magma flows through the fissure to form new oceanic crust. They form the longest mountain range in the world extending for more than 56,000 km long.